Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g. AlGaAs) have been developed since the mid-80's (Kinoshita et al., IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, June 1987). They have reached the point where AlGaAs-based VCSELs emitting at 850 nm are manufactured by a number of companies and have lifetimes beyond 100 years (Choquette et al., Proceedings of the IEEE, Vol. 85, No. 11, November 1997). With the success of these near-infrared lasers, attention in recent years has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (Wilmsen et al., Vertical-Cavity Surface-Emitting Lasers, Cambridge University Press, Cambridge, 2001). There are many potential applications for visible lasers, such as, display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (Ishigure et al., Electronics Letters, Vol. 31, No. 6, Mar. 16, 1995). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to be done to create viable laser diodes (either edge emitters or VCSELs) that produce light output that spans the visible spectrum.
In an effort to produce visible wavelength VCSELs it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems, since organic-based gain materials can enjoy a number of advantages over inorganic-based gain materials in the visible spectrum. For example, typical organic-based gain materials have the properties of low unpumped scattering/absorption losses and high quantum efficiencies. In comparison to inorganic laser systems, organic lasers are relatively inexpensive to manufacture, can be made to emit over the entire visible range, can be scaled to arbitrary size and, most importantly, are able to emit multiple wavelengths (such as red, green, and blue) from a single chip. Finally, organic lasers have a very large gain bandwidth, especially in comparison with inorganic lasers. Over the past number of years, there has been increasing interest in making organic-based solid-state lasers. The laser gain material has been either polymeric or small molecule and a number of different resonant cavity structures were employed, such as, microcavity (U.S. Pat. No. 6,160,828 issued Dec. 12, 2000 titled “Organic Vertical-Cavity Surface-Emitting Laser,” by Kozlov et al.), waveguide, ring microlasers, and distributed feedback (see also, for instance, Kranzelbinder et al., Rep. Prog. Phys. 63, 729-762, 2000 and U.S. Pat. No. 5,881,083 issued Mar. 9, 1999 titled “Conjugated Polymers As Materials For Solid State Laser”). A problem with all of these structures is that in order to achieve lasing it was necessary to excite the cavities by optical pumping using another laser source. It is much preferred to electrically pump the laser cavities since this generally results in more compact and easier to modulate structures.
A main barrier to achieving electrically pumped organic lasers is the small carrier mobility of organic material, which is typically on the order of 10−5 cm2/(V−s). This low carrier mobility results in a number of problems. Devices with low carrier mobilities are typically restricted to using thin layers in order to avoid large voltage drops and ohmic heating. These thin layers result in the lasing mode penetrating into the lossy cathode and anode, which causes a large increase in the lasing threshold (Kozlov et al., Journal Of Applied Physics, Vol. 84, Number 8, Oct. 15, 1998). Since electron-hole recombination in organic materials is governed by Langevin recombination (whose rate scales as the carrier mobility), low carrier mobilities result in orders of magnitude having more charge carriers than singlet excitons; one of the consequences of this is that charge-induced (polaron) absorption can become a significant loss mechanism (Tessler et al., Applied Physics Letters, Vol. 74, No. 19, May 10, 1999). Assuming laser devices have a 5% internal quantum efficiency, using the lowest reported lasing threshold to date of ˜100 W/cm2 (Berggren et al., Letters to Nature, Vol. 389, Oct. 2, 1997), and ignoring the above mentioned loss mechanisms would put a lower limit on the electrically-pumped lasing threshold of 1000 A/cm2. Including these loss mechanisms would place the lasing threshold well above 1000 A/cm2, which to date is the highest reported current density, which can be supported by organic devices (Tessler et al., Advanced Materials, 1998, 10, No. 1).
One way to avoid these difficulties is to use crystalline organic material instead of amorphous organic material as the lasing media. This approach was recently taken (Schon et al., Science, Vol. 289, Jul. 28, 2000) where a Fabry-Perot resonator was constructed using single crystal tetracene as the gain material. By using crystalline tetracene, larger current densities can be obtained, thicker layers can be employed (since the carrier mobilities are on the order of 2 cm2/(V−s)), and polaron absorption is much lower. Using crystal tetracene as the gain material resulted in room temperature laser threshold current densities of approximately 1500 A/cm2.
An alternative to electrical pumping for organic lasers is optical pumping by incoherent light sources, such as, light emitting diodes (LEDs), either inorganic (McGehee et al., Applied Physics Letters, Vol. 72, No. 13, Mar. 30, 1998) or organic (U.S. Pat. No. 5,881,089 issued Mar. 9, 1999 titled “Article Comprising An Organic Laser” by Berggren et al.). This possibility is the result of unpumped organic laser systems having greatly reduced combined scattering and absorption losses (˜0.5 cm−1) at the lasing wavelength, especially when one employs a host-dopant combination as the active media. Even taking advantage of these small losses, the smallest reported optically pumped threshold for organic lasers to date is 100 W/cm2 based on a waveguide laser design (Berggren et al., Letters to Nature, Vol. 389, Oct. 2, 1997). Since off-the-shelf inorganic LEDs can only provide up to ˜20 W/cm2 of power density, it is necessary to take a different route to avail of optically pumping by incoherent sources. Additionally, in order to lower the lasing threshold it is necessary to choose a laser structure that minimizes the gain volume; a VCSEL-based microcavity laser satisfies this criterion. Using VCSEL-based organic laser cavities should enable optically pumped power density thresholds below 5 W/cm2. As a result, practical organic laser devices can be driven by optically pumping with a variety of readily available, incoherent light sources, such as LEDs.
There are a few disadvantages to organic-based gain media, but with careful laser system design these can be overcome. Organic materials can suffer from low optical and thermal damage thresholds. Devices will have a limited pump power density in order to preclude irreversible damage to the device. Organic materials additionally are sensitive to a variety of environmental factors, like oxygen and water vapor. Efforts to reduce sensitivity to these variables typically result in increased device lifetime.
One of the advantages of organic-based lasers is that since the gain material is typically amorphous, devices can be formed inexpensively when compared to lasers with gain materials that require a high degree of crystallinity (either inorganic or organic materials). Additionally, lasers based upon organic amorphous gain materials can be fabricated over large areas without regard to producing large regions of single crystalline material; as a result they can be scaled to arbitrary size resulting in greater output powers. Because of their amorphous nature, organic-based lasers can be grown on a wide variety of substrates; thus, materials such as glass, flexible plastics, and Si are possible supports for these devices. Thus, there can be significant cost advantages as well as a greater choice in usable support materials for amorphous organic-based lasers.
Tunable inorganic VCSELs are well established in the art. A variety of tuning mechanisms have been described with a wide range of characteristics. Chang-Hasnain (IEEE J. Quant. Electr. 6, 978 (2000)) has recently reviewed advances in wavelength-tunable VCSELs. Micromechanical tunable inorganic VCSELs are emphasized in this article. Continuous wavelength tuning is a feature of micromechanical or microelectromechanical (MEM) means of tuning the wavelength output of solid state laser sources, in particular, inorganic VCSELs. A 15 nm tuning range is described in M. C. Larson and J. S. Harris, Appl. Phys. Lett. 68, 892 (1996) for an inorganic VCSEL with a micromachined, deformable-membrane mirror. With improvements in the movable mirror design, a 19.1 nm tuning range has been demonstrated (Sugihwo et al., Appl. Phys. Lett. 70, Feb. 3, 1997). The physical basis for such MEMs means of tuning is the changing of the optical path length of the laser cavity. The most straightforward method for changing of the optical path length of the laser cavity is movement of the laser cavity mirror. An early version of the use of this tuning mechanism for thin film lasers is described U.S. Pat. No. 3,373,654 issued Mar. 19, 1968 titled “Display Viewing Apparatus” by Carolan et al. More recently, the use of curved movable mirror elements is described for MEM-tunable inorganic VCSELs. Such structures offer improved control of lasing mode quality with single mode operation over a wide tuning range. In particular, U.S. Patent Application Publication Nos. 2002/0048301 (filed Apr. 5, 2000 by Wang et al.); 2002/0031155 (filed Jun. 26, 1998 by Tayebati et al.); and 2002/0061042 (filed Sep. 28, 2001 by Wang et al) provide detailed descriptions of the design of the movable mirror tuning structure.
MEMs devices have been used to tune inorganic laser cavities when used in combination with grating output coupler devices. John H. Jerman et. al. in U.S. Patent Application Publication No. 2001/0036206 (filed Mar. 12, 2001) describe the use of a microactuator to alter the angle of a micro-mirror used to tune a laser with a Littman-Metcalf configuration. The advantage of this particular configuration is that the output light direction is fixed as the wavelength is tuned. This particular cavity configuration is an example of an external cavity laser implemented with MEMs means of wavelength control. Such devices are referred to as MEM-ECL devices. The Littman-Metcalf configuration is but one example of a number of MEM-ECL device laser configurations described in this patent application Publication.
Alternative means for changing the optical path length of the laser cavity for thin film inorganic lasers have been described. These alternative means are generally non-mechanical in nature. The alternative means typically involve affecting the optical path length of a laser cavity via a change in the index of refractive index in one or more portions of the device. Inorganic VCSELs may be tuned by methods that are described as thermal or thermal-electric mechanisms. Both such mechanisms rely on the temperature dependence of the refractive index to affect an optical path length change within the laser cavity. Fan et. al., Electronics Letters, Vol. 30, No. 17, Aug. 18, 1994, reported a tuning range of approximately 10 nm in an inorganic VCSEL device that incorporated an integrated thin film heater. The tuning was accomplished by a completely thermal mechanism. A thermal-electric mechanism is described by Chang-Hasnain et. al. in Electr. Lett. 27, 11, p. 1002 (1991). In this case, an additional structure is incorporated into the inorganic VCSEL, that functions to control the temperature. With appropriate current flow in the device, the laser can be cooled via the Peltier effect leading to a blue wavelength shift of the laser emission. Under other conditions, the current causes a heating of the device and a subsequent red wavelength shift of the laser emission. Low-threshold devices tuned by the heating mechanism are described in Wipiejewski et. al., IEEE Photonics Technology Letters, Vol. 5, No. 8, August 1993. Additionally, there is a tuning mechanism that is a consequence of the concentration of free electronic carriers in the laser cavity, the so-called plasma effect (see, for example, Gmachi et al., Appl. Phys. Lett., 62 (3), January 1993). The carrier density causes a decrease of refractive index and a decrease of the laser output wavelength. The tuning range is quite limited in these devices. These methods also suffer from a limited frequency response for the modulation; the tuning rate is rather small.
A number of other methods of inorganic VCSEL wavelength control have been reported that rely upon refractive index changes to affect a tuning. E. A. Avrutin et. al. in Appl. Phys. Lett. 63(18), p. 2460 (1993) describe the incorporation of index-changing layer or layers in the distributed Bragg reflector (DBR) portion of the device. The DBR typically forms one of the end mirrors that define the laser cavity.
MEMs devices may be used to select the output of different laser oscillators as opposed to tuning laser cavities either by changing the optical path length of a laser cavity or affecting an angle change with a grating output coupler. A widely tunable laser module is described in B. Pezeshki et. al., Optical Fiber Communication Conference (OFC) Proceedings Technical Digest Postconference Edition, volume 70, March 2002, Optical Society of America. An array of multi-wavelength lasers produces light outputs at discrete wavelengths within a wavelength tuning range. The output of a single laser cavity device is selected by optics and a MEMs tilt mirror to direct the appropriate, selected wavelength output to an output optical fiber. Wavelength selection is accomplished by control of the MEMs tilt mirror angle within the system.
Kozlov et al., in U.S. Pat. No. 6,160,828 (Dec. 12, 2000) describe organic VCSEL devices with a capability for wavelength tuning. Like the inorganic material-based systems described above, the optical path length of the laser cavity is changed to affect wavelength tuning. Two different embodiments are described. In the first, the laser organic layer that provides optical gain, is in the form of a wedge or tapered layer. The thickness of the organic layer varies laterally in the device. Optically pumping different portions of the wedge device produces outputs at different wavelengths. Smooth tuning ranges for such organic devices are significantly greater than for inorganic devices; tuning ranges of 50 nm or more are reported. In an alternative embodiment, the second (top) mirror element is translated with respect to the rest of the device structure to produce an optical path length change. A lens is incorporated into the cavity to direct the light to the second mirror element. With both such devices it is difficult to control the lateral mode structure of the lasing emission, as the active volume in the cavity is only determined by the pump beam spot size. In the wedge device, the spectral width of the laser output is also sensitive to the pump beam spot size in such a device structure. Additionally, in the case where a lens is incorporated into the cavity, such an extended length cavity has many longitudinal modes. It is difficult to perform smooth cavity tuning in such structures. The addition of the lens adds cost and complexity to the system and complicates the optical alignment.
What is needed is better laser optical mode control and tuning wavelength control of organic tunable VCSELs while maintaining the great tuning range advantage of organic tunable VCSELs over inorganic VCSELs.